When a number is added to or subtracted from a function it moves it vertically up or down.
G(x) is obtained by subtracting 3 from F(x). This means G(x) is obtained by vertical translation of F(x). Since 3 is subtracted from F(x), the vertical translation is downwards.
So, we can say: G(x) is obtained by moving F(x) 3 units vertically down.
Therefore, the correct answer is option D